The present invention relates to the control of the power supply to discharge lamps, especially in motor vehicle headlights.
Reference is made here to FIG. 1 of the accompanying drawings, which shows diagrammatically a power supply device for a discharge lamp for a motor vehicle headlight. Conventionally, the power supply device includes a direct current/direct current converter 1 (referred to in this specification as a dc/dc converter), which feeds power to a second converter 2. The converter 2 is a direct current/alternating current converter (referred to in this specification as a dc/ac converter), which itself supplies the discharge lamp 3.
The dc/dc converter 1 includes for example a transformer T and an interrupter Q1, which controls the power supply to the primary transformer T, the interrupter Q1 being itself controlled by a regulator 4. Interruptors Q2, in the H-connected bridge constituting the dc/ac converter 2, are controlled by a control circuit 5 which is a clock circuit.
The switching frequency of the interrupters in the converter 1 is of the order of 200 Hz to 1 KHz. However, such low frequency power supply gives rise to a substantial amount of electromagnetic noise.
One object of the invention is to propose a control means which enables the above mentioned noise to be eliminated completely, or at least to be substantially reduced.
In the field of electronics, it is known in general terms that electromagnetic noise generated by a power circuit with a switching function can be greatly diminished by widening the spectrum of the control frequencies. In this regard, reference may be made to the following papers:
xe2x80x9cThe Effect of Carrier Frequency Modulation of PWM Waveforms on Conducted EMC Problems in Switched Mode Power Suppliesxe2x80x9d, Stone, David and Chambers, Barry, University of Sheffield, Department of Electronic and Electrical Engineering, EPE Journal volume 5 No. 3/4, January 1996;
xe2x80x9cEffect of Spread-spectrum Modulation of Switched Mode Power Converter PWM Carrier Frequencies on Conducted EMIxe2x80x9d, Stone, David and Chambers, Barry, University of Sheffield, Department of Electronic and Electrical Engineering, IEE Mar. 16, 1995, Electronics Letters No. 19950543;
xe2x80x9cRandom Carrier Frequency Modulation of EMC Problems in Switched Mode Power Suppliesxe2x80x9d, Stone, David, Chambers, Barry and Howe, David, University of Sheffield, Department of Electronic and Electrical Engineering, IEEE Catalogue No. 95TH8025 1995.
One solution for reducing the electronic noise produced by the alternating power supply of a discharge lamp could therefore be to modify the control of the interrupters of its dc/ac converter so as to widen its switching frequency spectrum.
According to the invention in a first aspect, a method of controlling a power supply device for a discharge lamp, especially for a motor vehicle headlight, the said power supply device comprising a dc/ac converter, is characterised in that the power supplied to the said dc/ac converter is modulated over time.
Preferably, the power supplied to the dc/ac converter is modulated in a pseudo-random sequence.
A method according to the invention has the advantage that it spreads the spectrum which leads to reduction or suppression of the electromagnetic noise without any modification of the control for the interrupters of the dc/ac converter.
According to the invention in a second aspect, a power supply device for a discharge lamp, especially for a motor vehicle headlight, comprising a dc/ac converter and means for generating a supply power which is passed as input to the said converter, is characterised in that the said means comprise means for modulating the said supply power.
Preferably, the means for generating the supply power passed as input to the said converter comprise a dc/dc converter and regulating means, and the said regulating means control the dc/dc converter in such a way that the latter delivers modulated power.
In another embodiment of the invention, the means for generating the supply power transmitted as input to the dc/ac converter comprise a dc/dc converter, and the said device further includes means for adding a modulated differential power to the power output from the direct current/direct current converter.